The present invention relates generally to the field of marine surveying. More particularly, in one or more embodiments, this invention relates to using a rigid-stem assembly comprising a plurality of interconnected rigid stems in a marine seismic survey.
Techniques for marine surveying include marine geophysical surveying, such as seismic surveying and EM surveying, in which geophysical data may be collected from below the Earth's surface. Geophysical surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Marine geophysical surveying is typically performed using one or more “streamers” towed below or near the surface of a body of water. The streamers are typically cables that include a plurality of sensors disposed thereon at spaced apart locations along the length of the cable. The sensors may be configured to generate a signal that is related to a parameter being measured by the sensor. An energy source may also be towed through the water by the same or a different vessel. At selected times, the energy source may be actuated to generate, for example, seismic or EM energy that travels downwardly into the subsurface rock. Seismic or EM energy that interacts with interfaces, generally at the boundaries between layers of rock formations, may be returned toward the surface and detected by the sensors on the streamers. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
In geophysical surveying, the streamer is typically a cable that is stored on a drum. The streamers are typically made of multiple components, such as electrical conductors, fiber optics, and stress-supporting members, all bundled together and covered with a protective outer skin. The streamer may be up to several kilometers in length. A lead-in is typically a cable that couples the streamer to the survey vessel. The lead-in may also be stored on a drum, typically the same drum as the streamer. To deploy, retrieve, and store the cables, cable-handling systems are employed that may include a storage drum in combination with sheaves, blocks, and other turns where a smaller angle of cable direction changes occurs are used. When one section of the streamer is taken off, repaired, or replaced, drums often with a smaller diameter are used. In operation, one of the drums is holding back the tension in the deployed cable with its turning moment. In order to avoid load concentration on the cable, it may be beneficial to have many rounds of cable inside the drum while applying force. Thus, the innermost drum, which is the storage drum, is often used to hold back the tension.
The drum causes a number of restrictions and costly features on the streamers, especially in combination with rigid elements such as connectors, electronics housing, and sensor spacers. For example, the streamer typically should be capable of handling large deformations as a result of storage on the drums, thus limiting the available alternatives for the outer skin of the streamer. Accordingly, options for treating the streamer for drag reduction and antifouling may be limited. In addition, certain streamers, such as gel-filled cables, may encounter large, undesired deformations in shape due to storage on the drum, preventing their re-use. For various reasons, some of the streamer components may be external to the cable and attached via connection points to the outside. These external components typically cannot be attached to the cable until the cable is unwound from the drum during deployment. To reduce the complexity associated with this attachment process, there has been an ongoing effort to place these components inside the outer skin of the cable. However, placement of the equipment inside the outer skin is not always practical as adapting a solution for incorporating these components into a cable that is wound onto a drum can be challenging.
Accordingly, there is a need for improved methods and systems for deploying and recovering streamers and other equipment in marine seismic surveys.